Re: [virtio] [PATCH 3/4] ccw: virtio revision vs. virtio feature bits

[Cornelia Huck <cornelia.huck@de.ibm.com>]

On Sun, 8 Feb 2015 11:21:18 +0100
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Fri, Oct 31, 2014 at 03:26:18PM +0100, Cornelia Huck wrote:
> > We don't want the VERSION_1 feature bit unless at least revision 1
> > has been negotiated. Make this a requirement.
> > 
> > VIRTIO-119
> > 
> > Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
> 
> Sorry about the delay in response.
> I think I disagree with this one.
> 
> I think this discusses how transitional devices work when
> using the legacy interface?
> In that case, this needs to go into a separate section,
> explicitly labelled "legacy".

Hmm... but a legacy device does not actually know about VERSION_1, does
it? Or am I misunderstanding what you mean?

> Looking into this deeper, how do devices "not accept"
> bits when using a legacy interface?

A legacy device can't accept the VERSION_1 bit, as it only has 32
feature bits. It will fail the ccw for the second half of the feature
bits.

> 
> Also, the wording here seems wrong to me.

Could you elaborate?

> 

> > @@ -2656,6 +2656,14 @@ For communicating its supported features to the device, the driver
> >  uses the CCW_CMD_WRITE_FEAT command, denoting a \field{features}/\field{index}
> >  combination.
> >  
> > +\devicenormative{\paragraph}{Handling Device Features}{Virtio Transport Options / Virtio over channel I/O / Device Initialization / Handling Device Features}
> > +
> > +The device MUST NOT offer VIRTIO_F_VERSION_1 if not at least revision 1 has
> > +been negociated.
> > +
> > +The device MUST NOT accept VIRTIO_F_VERSION_1 if not at least revision 1 has
> > +been negociated.

I think this all boils down to whether revision handling is enough to
designate a device as non-legacy, no?

> > +
> >  \subsubsection{Device Configuration}\label{sec:Virtio Transport Options / Virtio over channel I/O / Device Initialization / Device Configuration}
> >  
> >  The device's configuration space is located in host memory.